The present invention relates to an extrusion head assembly for extruding simultaneously a plurality of tubes of plastics material between the open halves of a mold for subsequent blow molding purposes.
It is a common procedure to use multiple extrusion heads with a screw extruder to form hollow tubes that can be blown in molds to the shapes of hollow articles. When this procedure is practiced using dual extrusion heads with accumulator chambers allowing intermittent extrusion, of the types shown, for example, in U.S. Pat. No. 3,909,183, or U.S. Pat. No. 3,611,494, it is common practice to use separate mold clamps and associated mold halves for operation below each extrusion head. The cycles of operation at the extrusion heads are then diametrically opposed, that is, extrusion will be occurring at one extrusion head while the mold halves associated therewith are open, and blowing will then be occurring at the other extrusion head at which the associated molds will be closed, and the accumulator chamber at this head will be receiving plastics material in the charging portion of the cycle. The principal reason for alternating the cycles of operation of the dual extrusion heads is to avoid numerous problems that otherwise are inherent in this type of operation when using prior art apparatus and procedures.
These problems relate to uneven supply of plastics material from the one screw extruder to the dual or multiple extrusion heads and their associated accumulator chambers, uneven discharge from the extrusion heads so that one extruded tube may be longer than another, irregular timing of discharge so that the tubes to be blown may have stretched to different lengths, and the like. These irregular operating conditions produce further undesirable results, such as that they slow the cycles of operation, cause greater waste of plastics material, cause a higher percentage of "rejects" to be blown, and similar results.
Thus, to overcome these problems, the blow molding industry alternates the cycles of operation of the dual extrusion heads, and when so doing, separate clamps and associated molds, as well as separate controls and similar features for each head are required, increasing equipment and operating costs.